Russian prison authorities have moved Pussy Riot musician Nadezhda Tolokonnikova to an isolation cell after she launched a hunger strike over what she called death threats and slave labour conditions.

Tolokonnikova, 23, is half-way through a two-year sentence after being convicted over punk band Pussy Riot's protest song in a Moscow cathedral last year.

"A decision has been taken to move her to a safe place," the prison service for the central Mordovia region, where the activist is being held in a penal colony, said in a statement.

It said she was in an "isolated room", adding that she was moved because of Tolokonnikova's claims that "she is being threatened".

Her husband Pyotr Verzilov confirmed the move to news agency AFP.

"She's in a secure place. It's not a comfortable place though," he said.

The prison service has insisted that Tolokonnikova's solitary cell had a "level of comfort [that] meets legal norms".

"She has her personal things, food, books and stationery," it said.

Tolokonnikova declared a hunger strike on Monday, saying in a letter released to the media that prisoners had to work 17-hour days sewing uniforms and that she received a death threat from the colony's deputy governor.

In her letter, Tolokonnikova described harrowing conditions, saying inmates were treated like "slaves" and that a culture of violence saw prison staff encouraging inmates to beat up rule-breakers.

She alleged prisoners were forced to work naked as punishment for sewing slowly and described insanitary conditions, with the women often deprived of weekly showers and having to unblock overflowing toilets themselves.

In the most serious allegation, she said the camp's deputy governor hinted she would be killed by inmates if she pushed for their rights.

He warned that if the prisoners worked shorter hours, they would be punished for failing to reach production targets and would take revenge on her.

"When they find out this happened because of you, then nothing bad will happen to you again - because nothing bad happens in the afterlife," Tolokonnikova accused him of saying.

The regional investigative committee said it had launched a preliminary probe into her claims.

More than 22,000 people have signed an online petition asking the prison service to move Tolokonnikova to another prison colony.

The regional prison service has denied Tolokonnikova's claims, saying prisoners only work an eight-hour day and that Tolokonnikova made the complaint after being refused a request for privileges.

The other imprisoned Pussy Riot punk, Maria Alyokhina, 25, has also complained of rights abuses at her camp in the Urals region and has spent much of her sentence in voluntary isolation.